Our genetic makeup. Just like all other social animals. "Cain't we all jes' git alaong?" (Rodney King). No, because everyone wants to mind everyone else's business. Pick a subject -- religion, sex, economics... Everyone's functioning depends on them thinking they are right. And the more insecure people are, the more they want to mind others' business for them. E.g., Christian and Muslim fundamentalists.
Caesar, this is not proper.
CAESAR (recovering his self-possession):
Pardon him, Theodotus; he is a barbarian, and thinks that the customs of his tribe and island are the laws of nature.
Caesar and Cleopatra, Act II -- [George Bernard Shaw]